The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.
For example, carrier mobility is an important concern for the performance of a transistor, such as a metal oxide field effect transistor (MOSFET). With its decreased size, a transistor also has a decreased channel length, making it easier for impurities from source and drain regions of the transistor to diffuse into its channel region. Such impurities consequently reduce mobility of the carriers within the channel region. This is particularly troublesome with p-type MOSFETs where boron is usually the dopant in the source and drain regions because boron has a lower atomic weight and longer diffusing length than other commonly used dopants, such as phosphorus, in n-type MOSFETs. Furthermore, it has been observed that there are higher variations in ion implantation depth with smaller transistors. This contributes to higher variations in both carrier mobility and threshold voltage (Vt) of such transistors, adversely affecting their performance.